1. Field of the Invention
This invention relates to the preparation of polymer foams and is more particularly concerned with novel catalyst combinations for the trimerization of polyisocyanates to polyisocyanurate foams.
2. Description of the Prior Art
Rigid polyisocyanurate foams having resistance to flame and heat as well as excellent thermal insulating capacity are known in the art. The prior art discloses methods for preparing such foams by reacting an organic polyisocyanate with a trimerizing catalyst in the presence of a blowing agent, and a minor proportion (usually less than 0.5 equivalent per equivalent of polyisocyanate) of a polyol; see for example U.S. Pat. Nos. 3,516,950, 3,580,868, 3,620,986, 3,625,872, 3,635,848, 3,725,319, 3,745,133, G. B. Pat. No. 908,337, and DBP 1,112,285.
The above methods require the foaming process to proceed in conjunction with at least two polymer forming reactions; namely, the isocyanurate formation arising from the homopolymerization of the isocyanate employed, and the minor amount of polyurethane formation arising from the reaction of a polyol with the isocyanate. Difficulties arise, particularly in commercial applications, due to the variation in relative rates of the two polymerization reactions. The polyurethane formation frequently initiates before the trimerization formation thus giving two separate rise steps in the overall foam rise profile.
These difficulties are accentuated in the case of application of polyisocyanurate foams wherein rapid rise characteristics are desirable, such as in spray, or laminate foam systems. The result of simply increasing the concentration of catalysts of the prior art in an attempt to achieve rapid rise characteristics gives rise to poor conversion to polyisocyanurate and foam shrinkage. It has not hitherto been possible to vary the foam rise characteristics over a range of rapid rise times while maintaining an efficient trimerization reaction throughout the range, in the absence of foam shrinkage.
It is an object of the present invention to provide a catalyst combination which is free from the difficulties described hereinbefore, which combination will be set forth in detail hereinbelow. The catalyst combination finds particular utility where a variable range of rapid rise times is desired and thereby accomplished by simple adjustment in catalyst concentration.